Grünberg Bob, Emmler Thomas, Gedat Egbert, Shenderovich Ilja, Findenegg Gerhard H, Limbach Hans-Heinrich, Buntkowsky Gerd
Freie Universität Berlin, Institut für Chemie, Takustrasse 3, 14195 Berlin, Germany.
Chemistry. 2004 Nov 5;10(22):5689-96. doi: 10.1002/chem.200400351.
The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), pore filling by pore condensation (axial-pore-filling mode) occurs at a low relative pressure, corresponding roughly to a single adsorbed monolayer. For SBA-15, owing to its larger pore diameter (8 nm), a gradual increase in the thickness of the adsorbed layer (radial-pore-filling mode) prevails until pore condensation takes place at a higher level of pore filling.
采用1H MAS（MAS = 魔角旋转）和静态固态核磁共振光谱研究了具有均匀直径圆柱形孔的两种介孔二氧化硅材料MCM - 41和SBA - 15中的水吸附情况。所有观察到的氢原子要么是表面 -SiOH基团，要么是氢键结合的水分子。与MCM - 41不同，SBA - 15的内表面存在一些强结合水分子，这些水分子归因于表面缺陷。在更高的填充水平下，观察到MCM - 41和SBA - 15之间的另一个差异。MCM - 41中的水分子表现出化学位移的双峰线分布，一个峰位于内部本体水的位置，第二个峰位于与表面 -SiOH基团快速交换的水分子的位置。在SBA - 15中，观察到一条单线，随着孔填充量的增加，该单线连续移动。该结果归因于两种二氧化硅材料不同的孔填充机制。在MCM - 41中，由于其孔径较小（3.3 nm），通过孔冷凝进行的孔填充（轴向孔填充模式）在低相对压力下发生，大致对应于单个吸附单层。对于SBA - 15，由于其孔径较大（8 nm），在孔填充量更高时发生孔冷凝之前，吸附层厚度逐渐增加（径向孔填充模式）占主导。
